1. The Field of the Invention
The present invention relates to upflow bioreactors for decomposing organic materials. More particularly, the present invention relates to pressure release mechanisms for upflow bioreactors that have an aperture that can become plugged.
2. The Related Technology
A bioreactor is a device that uses bacteria to promote the decomposition or “digestion” of organic waste material into simple organics and gaseous biogas products. Biogas is typically a mixture of methane, carbon dioxide, hydrogen sulfide, and other volatile organic compounds. If produced in sufficient quantities, the methane gas can be used as a fuel.
Anaerobic digestion in open storage vessels has historically been utilized in waste water management, especially in livestock production, to reduce or convert complex organic matter to a smaller volume. This method has proven to be economical by reducing the volume of waste handled and by volatilizing some metabolites into the atmosphere. Some disadvantages of the process include poor ability to keep the anaerobic digestion process in balance, resulting in the release of malodorous gases and inefficient, incomplete conversion of digested organic matter into biogas. The slow rate of digestion and the poor quality of methane gas yielded makes the economic recovery of methane gas generally infeasible in open storage vessels.
Many attempts have been made to decompose organic waste using closed vessels. One type of closed vessel reactor that has shown very high decomposition rates is the upflow anaerobic sludge blanket reactor. In this reactor, waste material is introduced into the bottom of the reactor and forced up through the vessel where it passes through a blanket of bacteria, which decomposes the organic material and produces biogas, which can be collected and used as a fuel.
To achieve high decomposition rates in an upflow bioreactor, the bacterial culture should be well established. One important advantage of an upflow bioreactor is that it can be operated continuously. Thus, once the bacterial culture is established, the high rate of digestion can be maintained for an extended period of time (e.g. months or even years).
Recently, an upflow reactor has been developed that induces formation of the bacterial culture on startup and maintains a thicker or more enriched bacterial culture during operation. These benefits are achieved by placing a septum near the top of the fluid level of the reactor. The septum causes suspended solids to settle out of the fluid nearing the top of the reactor. These solids settle back into the biomass where digestion continues. The solids that are retained by the septum often include bacteria. By retaining these solids in the digester, the septum maintains a better bacterial culture and facilitates more complete digestion. Upflow reactors with a septum are capable of significantly reduced digestion retention times.
One problem with using a septum in an upflow bioreactor is the increased chance of plugging. To allow the effluent to exit the digester, the septum has an aperture. The aperture constricts flow and is thus susceptible to plugging. If the aperture plugs, extremely high pressure can build in the lower chamber as the bacteria continue to produce biogas.
The size of the aperture significantly impacts whether the aperture will plug. Although a larger diameter aperture would reduce the incidence of plugging, an aperture with a diameter sufficiently large to prevent all plugging would significantly diminish the ability of the septum to retain suspended solids.